sine curve. To draw such a curve we may proceed as follows: Draw, as shown in Fig, 20, a circle with a radius equal to the crest value of the current in any convenient scale. Divide out the circle into a number of equal parts, and divide the line a b in Fig. 19 into the same number of equal parts. Now let the radius rotate, and every time it comes to one of
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the points marked out on the circle, measure the height of this point over the horizontal, and plot this height over the horizontal in Fig. 19 at the corresponding division point. By the time we have once gone round the circle, we shall have obtained, in Fig. 19, all the points of the sine curve between a and b which are required to draw this curve. The rotating radius is called a "vector," in this case a "current vector,". since its projection on a vertical line gives at any moment during its
